Clothes, tone, and body movements are three of the most important things when it comes to conveying a message to an audience in the correct way. If you're doing a speech where you're celebrating someones achievements, you do not want to wear goofy clothes with a somber tone. Excitement is needed at such a time where celebration is had. Goofy clothes bring away from the person being celebrated and the somber tone is boring and unfit for such an occasion.
Correct, if would be quite bad if you wore a colorful suit to a funeral. And it would also be bad if you wore grieving clothes to a birthday party. Knowing your audience and the context will help you be prepared.
I agree especially with how your posture is when speaking. Great example with the celebratory speech.